1. Field of the Invention
The present invention generally relates to an amplifier circuit and an analog-to-digital converter using the same, and it particularly relates to an amplifier circuit which inputs selectively a plurality of analog signals via a switch or switches and an analog-to-digital converter utilizing the amplifier circuit.
2. Description of the Related Art
In recent years, systems employing a switched-capacitor type amplifier circuit, including analog-to-digital digital converters (hereinafter referred to as “AD converter”) using a switched-capacitor type amplifier circuit, are often used in mobile devices such as cellular phones. And those mobile devices are provided with an increasing variety of added functions, such as image shooting function, image playback function, moving image shooing function and moving image playback function. Along with this trend, there is a growing demand for high-speed operation of those switched-capacitor type amplifier circuits.
On the other hand, there is also demand for smaller sizes of AD converters. One known related art to meet such a demand is an AD converter which comes with a cyclic stage capable of feeding the output of its own stage back to the input thereof (see Reference (1) in the following Related Art List, for instance).
Related Art List
(1) Japanese Patent Application Laid-Open No. Hei4-26229.
In FIG. 1 of the above-mentioned Reference (1), it is assumed that a subtracter circuit SUB1 having an amplifying function in the preceding stage receives an input selectively from a sample-and-hold circuit S/H1 and a DA converter DA2 through a switch (not shown). A subtracter circuit SUB2 having an amplifying function in the subsequent stage is also assumed to operate the same way. A sample-and-hold circuit S/H2 in the subsequent stage receives an input selectively from the preceding stage and from a sample-and-hold circuit S/H4 in its own stage through a switch SW1. Such a switch, which presents a large resistance component, increases the time constant of the circuit as a whole and causes delays in signal output.